1. Field of the Invention
The invention relates to a semiconductor device and a method for forming the same. In particular, it relates to a semiconductor device in which a semiconductor chip is molded into a package and a method for forming the same.
2. Description of the Related Art
The surface mount device (SMD) which is a conventionally known semiconductor device packaged as a surface mounting type is described with reference to the figures. A semiconductor device shown in FIG. 15 is a device called the quad flat package (QFP) type. The QFP is a plastic package in a fixed form, in which outer leads working as external terminals extend from the side surface of the package and are fixed to a mounting substrate by using a solder. The QFP type device is constituted as follows; a semiconductor chip 21 is fixed to an island (die pad) 20 formed on a metal frame which is the same as an outer lead 24 by using an epoxy-conductive paste for die bonding; an electrode pad 23 formed on the surface of the fixed semiconductor chip 21 is connected to an inner lead 24a connecting to the outer lead 24 by using a metal wiring 22 such as Au; and the semiconductor chip 21, inner lead 24a and metal wiring 22 are molded with a resin 25.
FIG. 16 shows a ball grid alley (BGA) type semiconductor device. The BGA comprises solder balls on the whole bottom surface of the package used as external terminals, in which the package enables the outer lead pitch of a QFP to be provided with a large number of pins generally having a fine pitch of 0.3 to 0.5 mm. The BGA type device is constituted as follows; a semiconductor chip 31 is fixed on a resin substrate 34 by using the epoxy-conductive paste for die bonding; the resin substrate 34 is provided with wiring layers 38 and 39 on the front and the back surface thereof; the front surface wiring layer 38 is connected to the wiring layer on the back surface 39 through a through hole electrode 36; an electrode pad 33 is formed on the surface of the semiconductor chip 31 and connected to the front surface wiring layer 38 on the resin substrate 34 by using a metal wiring 32; the back surface wiring layer 39 is provided with a solder ball 37 as an external electrode at a desired position; and the semiconductor chip 31 is encapsulated with a resin 35 using a transfer mold or potting. As described above, the electrode pad 33 on the surface of the semiconductor chip 31 is connected to the external electrode through the metal wiring 32, front surface wiring layer 38, through hole electrode 36 and back surface wiring layer 39.
When the package of a QFP or a BGA is used, the electrode pad formed on the surface of the semiconductor chip and outer lead is required to be maintained with a certain distance because the output is transferred from the electrode of the semiconductor chip by a bonded wiring. Therefore, the size of the package is greater than the semiconductor chip. Further, since the electrode pad must be formed in the periphery of the semiconductor chip, there is a drawback due to the necessity for forming the semiconductor chip itself of increased size.
Further, in the package of a BGA the radiation activity is lower compared with the case where only the mold resin is used for packaging because the resin substrate is used as a mounting substrate. Moreover, there is a problem of having the resin substrate curved because only a surface of a resin substrate mounted with the semiconductor chip is molded with the resin.